Method and device for determining the gas properties of a combustible gas

ABSTRACT

The invention relates to a method and a device to determine the gas properties of combustible gas, in particular of natural gas, wherein  
     a) at least part of the combustible gas is exposed to infrared radiation and the amount of infrared radiation absorbed by the combustible gas is determined for two wave lengths or spectral ranges, the two wave lengths or spectral ranges being selected so that the amounts of individual components of the combustible gas in their different percentages have an effect on the amounts absorbed and recorded,  
     b) the thermal conductivity is recorded and  
     c) the gas properties are determined from the three measurands.  
     The term gas properties is understood to mean the gas composition, the gross heating value, the Wobbe number, the normal density and the methane number.

BACKGROUND IF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a method and a device for determiningthe gas properties of a combustible gas. The term gas properties isunderstood to mean the gas composition, the gross heating value, theWobbe number or Wobbe index, the normal density and the methane number.

[0003] 2. Description of the Prior Art

[0004] The gross heating value can be the molar, mass-based orvolume-based gross heating value. The gross heating value of natural gasmust be measured when the gas is handed over from the supplier to thecustomer so that it can be billed. For example, in practice calorimetersor gas chromatographs are used to determine the gross heating value atdelivery stations where gas changes hands between two gas supplycompanies. When gas chromatographs are used, the gas composition isanalysed. Once the gas composition has been established, the grossheating value of the combustible gas can be determined on the basis ofthe gross heating value for the pure substances. When gas meters,particularly turbine meters, are used, the volume flow rate is measured.The volume flow rate must be converted with the aid of thecompressibility number from the operating condition to the normalcondition. The compressibility number can be calculated with the knownSGERG method (ISO 12213) from the gross heating value, normal densityand amount of CO₂. If the gross heating value is determined using acalorimeter, the normal density and the amount of CO₂ must also bemeasured. If a gas chromatograph is used, the normal density and amountof CO₂ can be calculated from the gas composition. The amount of energyis determined as the product of the gross heating value and the standardvolume flow rate.

[0005] The methods for determining the gross heating value which usecalorimeters or gas chromatographs provide very good results but thetechnology is complicated and this means very high investment andmaintenance costs. Such methods are too complicated and have too slow aresponse time for some industrial applications, in particular forcontrol and regulating purposes.

[0006] Correlative methods are also used to determine the gross heatingvalue or the amount of energy when gas transported under high pressureis to be billed. These correlative methods measure several physical orchemical variables and then calculate the gross heating value.

[0007] The DE 197 36 528 and the DE 198 08 533 teach correlative methodswhere the velocity of sound and the dielectric constant of thecombustible gas are measured as the input variables. The gross heatingvalue or the gas composition are calculated from these measurementsignals.

[0008] The velocity of sound may be measured using an ultrasonicflowmeter. Such meters, which are mainly used in the high-pressuresector, are, however, comparatively expensive. More reasonableultrasonic flowmeters have been developed for the residential sector.However, these meters have so far failed to successfully compete withthe conventional diaphragm meter. Therefore, it is by no means certainthat ultrasonic flowmeters will continue to be available for theresidential sector in future. The dielectric constant must be determinedwith a measuring instrument developed especially for this purpose.Therefore, the cost of such measuring equipment is relatively high.

[0009] It is necessary to know the properties of a gas, in particularthe gross heating value or the Wobbe number, for various industrialapplications, in particular for control purposes.

[0010] The Wobbe number or the Wobbe index is the quotient of thevolume-based gross heating value and the square root of the specificgravity of the gas. The Wobbe index is used in industry to control ormaintain the amount of energy supplied to gas consumers. A simplecorrelative method has yet to be developed for such purposes.

[0011] The methane number is an important factor for the operation ofgas engines. The methane number is a measure of the knock-resistance ofgaseous fuels. The methane number expresses the volume percentage ofmethane in a methane/hydrogen mixture which, in a test engine understandard conditions, has the same tendency to knock as the fuel gas tobe examined. If, for example, a natural gas has a methane number of 85,this means that, under certain engine conditions, this natural gas hasthe same tendency to knock as a mixture of 85% methane and 15% hydrogen.When the methane number is known, appropriate action can be taken toprevent the undesired knocking of gas-driven piston engines.

[0012] DE-A-19650302 teaches a method to determine the methane number.The fuel gas is exposed to infrared radiation. The amount of infraredradiation absorbed by the gas mixture is measured using a radiationdetector and the methane number of the fuel gas determined therefrom.

[0013] The methane number is determined by means of an optical filterwhich covers a section of the absorption spectrum in which thehydrocarbons contribute to absorption in a ratio which is virtuallyproportional to the methane number of the natural gas. The method isrelatively simple to use because the components of the infrared sensorscan be obtained at a reasonable price and the infrared detectors providean extremely accurate measurement signal and are easy to use inpractice.

[0014] With the state-of-the-art methods it has so far not beentechnically possible to determine the gross heating value of naturalgases by means of infrared absorption. The different natural gases mayalso contain nitrogen in addition to hydrocarbons such as methane,ethane, etc. Depending on the absorption spectrum filtered, the infraredsignal reacts very sensitively to volume percentages of hydrocarbons andto volume percentages of carbon dioxide but not to volume percentages ofnitrogen. This leads to unacceptable measuring inaccuracies since thevolume percentage of nitrogen in the natural gas fluctuates greatly andhas a great influence on the gross heating value.

SUMMARY OF THE INVENTION

[0015] Thus an object of the present invention is to provide a methodfor determining the gas properties of a combustible gas, in particularthe gross heating value, the Wobbe number and the methane number, whichdoes not involve burning the gas, is simple to use and offers sufficientaccuracy for billing and controlling purposes. A further object of thepresent invention is to create a simple measuring arrangement which canbe used under practical conditions.

[0016] With the inventive method to determine the gas properties, atleast part of the natural gas is exposed to infrared radiation and theamount absorbed by the natural gas is recorded by an infrared sensor foreach of two wave lengths or spectral ranges. In addition, the thermalconductivity is measured using a thermal conductivity sensor.

[0017] It is important that the three measurands react very differentlyto the different components and do not correlate. Thus, the individualcomponents or individual groups of components have each a first degreeof influence on the first measurand and a second degree of influence onthe second measurand, wherein the first and second wave lengths orspectral ranges are chosen so that the ratio of the first degrees ofinfluence is different to the ratio of the second degrees of influence.E.g. it is possible to determine directly the molar fraction of carbondioxide in the natural gas using one of the infrared sensors. Thissensor operates preferably at a wave length of approx. 4.3 μm. Thesecond infrared sensor, may detect the hydrocarbons in the natural gas.It preferably operates at a wave length of 3.5 μm. The wave length wasselected so that the sensor is as sensitive as possible to hydrocarbons,namely particularly to ethane, propane and butane. The thermalconductivity is most sensitive to nitrogen. The gas properties arecalculated from the signals of the infrared sensors, i.e. the twomeasured values for the percentage absorbed by the natural gas, and fromthe signal from the thermal conductivity sensor.

[0018] It has proven that the combination of the two signals from theinfraered sensors and the signal from the thermal conductivity sonsorprovides a very accurate determinatin of the gas properties for a greatvariety of natural gas. The inclusion of the signal from the thermalconductivity sensor causes a rapid increase of the accuracy incomparison to a method or apparatus which relies only upon the signalsfrom the infrared sensors.

[0019] The advantage of the inventive method is that conventionalsensors can be used for the measurements. The sensors are produced inlarge numbers in series production and are therefore very reasonablypriced and reliable. Furthermore, the sensors are very compact so thatthey can easily be installed in a common housing, for example, a 19″plug-in unit. As the gas passes directly through the sensors and thesensors have a very small volume, the response time is extremely quick,which is very important above all in the control of combustionprocesses.

[0020] The method described hereinabove covers all the different uses ofgas property determination described hereinbefore, i.e. energymeasurement (gross heating value, normal density, percentage of CO₂) andprocess control (Wobbe number/methane number) at the same time. Theaccuracy is comparable with the accuracy of the calorimeters or processgas chromatographs previously used for billing. The method describedhereinabove is, however, much cheaper and the maintenance costs are muchlower.

[0021] In principle, different types of sensors can be used to determinethe different measurands. However, each type of sensor provides its owntype-specific measurement values. Experiments have shown that a simplegas property correlation can be derived from the sensor signals. Inparticular two empirical relationships are used to establish thecorrelation. These two empirical relationships were established usinglaboratory measurements on methane and a number of natural gases.Firstly, the functional relationship between the gross heating valueH_(CH) of the hydrocarbons and the quotient of infrared absorption A andthe molar fraction x_(CH) of the hydrocarbons was established.

[0022] Furthermore, the thermal conductivity λ_(CH) of the hydrocarbongas is described as a function of the quotient of infrared absorption Aand the molar fraction x_(CH) of the hydrocarbons. The characteristiccurves only have to be established once for a certain sensor type. Forany subsequent calibration it is sufficient to randomly check with apure gas such as methane.

[0023] The accuracy of the method can be increased if the amount ofinfrared radiation absorbed by the combustible gas is determined for anadditional wave length of roughly 7.9 μm. At this wave length, thesensor is particularly sensitive to the volume percentage of methane inthe combustible gas. Furthermore, with this additional measurement it ispossible to set up a redundant system for testing purposes.

[0024] The amount of infrared radiation and the thermal conductivity arepreferably recorded under reference conditions in a common measurementenvironment.

[0025] The temperature and the pressure are preferably recorded or keptconstant in step a) or b).

[0026] The invention is further characterised in that the gas propertiesare determined according to the formulae (6), (3), (1), (7) (8.1-8.9)and (9) in accordance with FIG. 3.

[0027] The formulae (3), (4) and (6) contain the coefficients a₀, a₁, a₂and c₀, c₁, c₂, which are determined only once from the measurementsobtained from the process steps a) and b) on reference gases of knowngas composition or gas properties.

[0028] Normally three or more reference gases are measured for thispurpose. The coefficients are then determined by adjusting to thereference gases by minimising the residual sum of squares by linearregression. The basic calibration is performed only once for a piece ofequipment. For recalibration purposes it is sufficient to perform ameasurement with only one reference gas, e.g. pure methane (one-pointcalibration). With this one-point calibration only the coefficients a₀and c₀ are adjusted.

[0029] The invention also covers a method for determining the amount ofenergy in a combustible gas, in particular natural gas, characterised inthat the gross heating value is determined, the combustible gas ispassed through a volume flowmeter and the volume flow rate is measured.

[0030] The invention further relates to a device to determine the gasproperties of a combustible gas, in particular natural gas,characterised in that the natural gas is passed through an arrangementof sensors which mainly consists of a source of radiation for infraredradiation and at least two radiation detectors assigned to the source ofradiation as well as a sensor to record the thermal conductivity, andthat the signals of the arrangement of sensors are transmitted to anevaluation unit in which the gas properties are determined by means of acorrelation.

[0031] The invention is explained in more detail in the following withthe aid of a preferred embodiment and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0032] The drawing shows in:

[0033]FIG. 1: the molar gross heating value H_(CH) of the hydrocarbongas as a function of the quotient of infrared absorption A and the molarfraction x_(CH) of the hydrocarbons for 8 natural gases as well asmethane;

[0034]FIG. 2: the thermal conductivity λ_(CH) of the hydrocarbon gas asa function of the quotient of infrared absorption A and the molarfraction x_(CH) of the hydrocarbons for 8 natural gases as well asmethane;

[0035]FIG. 3: a calculation procedure to determine the gas compositionand the gas properties (gross heating value, Wobbe number, normaldensity, methane number) from the thermal conductivity λ, molar fractionof carbon dioxide x_(CO2), which results directly from the measurementof infrared absorption at a wave length of approx. 4.3 μm and infraredabsorption of the hydrocarbons A;

[0036]FIG. 4: a comparison of gas properties (gross heating value, Wobbenumber, normal density, methane number) determined according to theinventive procedure with values which have been obtained through gaschromatographic analysis;

[0037]FIG. 5: the results of a field test. The gross heating valuemeasured with the inventive device and with a calorimeter over a periodof 1 month is shown.

[0038]FIG. 6 a schematic of the arrangement of measuring instruments todetermine the gas properties of natural gases.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The following describes the calculation procedure and thecorrelation method according to FIG. 3.

[0040] The thermal conductivity λ, the molar fraction of carbon dioxidex_(CO2) which results directly from the measurement of infraredabsorption at a wave length of approx. 4,3 μm and the infraredabsorption of hydrocarbons A are measured as input variables.

[0041] Natural gas consists substantially of nitrogen, carbon dioxide aswell as a hydrocarbon gas, hereinafter referred to as CH, which ismainly composed of the alkanes methane to octane. As the amount ofnitrogen and the amount of carbon dioxide have no influence on the grossheating value, the molar gross heating value H_(s,m) of the natural gasresults from the molar fraction x_(CH), the gross heating value H_(CH)(H_(CH)=Σx_(CHi)·H_(CHi)) of the hydrocarbon gas:

H _(s,m) =x _(CH) ·H _(CH)  (1)

[0042] The molar fraction of the hydrocarbon gas is as follows:

x _(CH)=1−x _(N2) −x _(CO2)  (2)

[0043] As shown in FIG. 1, the gross heating value of the hydrocarbongas H_(CH) can be shown as a function of the quotient of infraredabsorption of the hydrocarbons A and the molar fraction of thehydrocarbons x_(CH):

H _(CH) =a ₀ +a ₁·(A/x _(CH))+a ₂·(A/x _(CH))²  (3)

[0044] This can be explained by the fact that the molar fractions of thealkanes are distributed regularly in the natural gas. The infraredabsorption A in equation (3) is measured with the infrared sensor at awave length of 3,5 μm.

[0045] The thermal conductivity λ_(CH) of the hydrocarbon gas can alsobe shown in a similar manner as a function of the quotient (A/x_(CH)).This relationship is shown in FIG. 2.

λ_(CH) =c ₀ +c ₁·(A/x _(CH))+c ₂·(A/x _(CH))²  (4)

[0046] The thermal conductivity λ of the natural gas can be shown as afunction of the molar fractions x_(N2), x_(CO2) and x_(CH) as follows:

λ=x _(N2)·λ_(N2) +x _(CO2)·λ_(CO2) +x _(CH)·λ_(CH)  (5)

[0047] The molar fraction of the hydrocarbon gas x_(CH) can be directlyderived from the measurands x_(CO2), A and λ by inserting equation (4)in equation (5) and solving for x_(CH). $\begin{matrix}\begin{matrix}{x_{CH} = \quad {\sqrt{{\frac{1}{4}\left\lbrack \frac{{c_{1} \cdot A} - {{\lambda x}_{CO2}\left( {\lambda_{CO2} - \lambda_{N2}} \right)} + \lambda_{N2}}{c_{0} - \lambda_{N2}} \right\rbrack}^{2} - \frac{c_{2} \cdot A^{2}}{\left( {c_{0} - \lambda_{N2}} \right)}} -}} \\{\quad {\frac{1}{2} \cdot \frac{{c_{1} \cdot A} - \lambda + {x_{CO2}\left( {\lambda_{CO2} - \lambda_{N2}} \right)} + \lambda_{N2}}{c_{0} - \lambda_{N2}}}}\end{matrix} & (6)\end{matrix}$

[0048] Values for the thermal conductivity of the pure substances λ_(N2)and λ_(CO2) can be found in literature.

[0049] The gross heating value H_(CH) of the hydrocarbon gas can becalculated from the molar fraction of the hydrocarbon gas using equation(3) and then the gross heating value of the natural gas from equation(1).

[0050] The molar fraction of the nitrogen x_(N2) can then be determinedas follows from the molar fractions x_(CH) and x_(CO2).

x _(N2)=1−x _(CH) −x _(CO2)  (7)

[0051] The relative densities of the individual alkanes from ethane tooctane can be derived from the gross heating value H_(CH) and the molarfractions x_(CH) of the hydrocarbon gas as follows:

x _(C2H6)={α₁(H _(CH) −H _(CH4))+β₁(H _(CH) −H _(CH4))²}  (8.1)

x _(C3H8)={α₂(H _(CH) −H _(CH4))+β₂(H _(CH) −H _(CH4))²}  (8.2)

x _(n−C4H10)={α₃(H _(CH) −H _(CH4))+β₃(H _(CH) −H _(CH4))²}  (8.3)

x _(i−C4H10)={α₄(H _(CH) −H _(CH4))+β₄(H _(CH) −H _(CH4))²}  (8.4)

x _(n−C5H12)={α₅(H _(CH) −H _(CH4))+β₅(H _(CH) −H _(CH4))²}  (8.5)

x _(i−C5H12)={α₆(H _(CH) −H _(CH4))+β₆(H _(CH) −H _(CH4))²}  (8.6)

x _(n−C6H14)={α₇(H _(CH) −H _(CH4))+β₇(H _(CH) −H _(CH4))²}  (8.7)

x _(n−C7H16)={α₈(H _(CH) −H _(CH4))+β₈(H _(CH) −H _(CH4))²}  (8.8)

x _(n−C8H18)={α₉(H _(CH) −H _(CH4))+β₉(H _(CH) −H _(CH4))²}  (8.9)

[0052] The coefficients α₁ to β₉ are determined only once using theanalysis of several reference gases with a known gas composition or gasproperties. The coefficients are determined by adjusting to thereference gases by minimising the residual sum of squares by linearregression.

[0053] The molar fraction of the methane is then as follows:

x _(CH4) =x _(CH) −x _(C2H6) −x _(C3H8) −x _(n−C4H10) −x _(i−C4H10) −x_(n−C5H12) −x _(n−C6H14) −x _(n−C7H16) −x _(n−C8H18)  (9)

[0054] The gas analysis of a total of 12 components (N₂, CO₂, 10alkanes) thus determined can now be used to derive further gasproperties such as the volumetric gross heating value H_(s), the Wobbenumber W_(s), the normal density ρ_(n) or the methane number MN. H_(s),W_(s) and ρ_(n) are calculated according to the international standardISO6976. A schematic of the calculation procedure is shown in FIG. 3.

[0055] The calculation procedure described contains the coefficients a₀,a₁, a₂ and _(c0), c₁, c₂, which are determined by a single basiccalibration. Calibration is performed by measurements (process steps a)and b)) on reference gases whose gas composition or gas properties areknown. Normally three or more reference gases are measured for thispurpose. The coefficients are determined by adjusting to the referencegases by minimising the residual sum of squares by linear regression.This basic calibration is performed only once for a piece of equipment.It is sufficient for any subsequent recalibration to perform ameasurement with only one reference gas, e.g. pure methane (one-pointcalibration). With this one-point calibration, only the coefficients a₀and c₀ are then adjusted.

[0056] The method was tested in the laboratory on a total of 8 differentnatural gases. FIG. 4 shows the procentual deviations of the gasproperties (gross heating value, Wobbe number, normal density andmethane number) measured with the arrangement of sensors and the valuesderived from gas chromatographic analysis. Generally, the deviations forthe gross heating value are less ±0.2%. Only with one gas sample was thedeviation 0.6%.

[0057] The long-term stability was examined in a field test where themeasurement signals were continually recorded and compared with acalorimeter which is used for billing purposes. The result is shown inFIG. 5 for a period of 1 month. The figure shows that the correlation ofthe gross heating value derived from the arrangement of sensors and thegross heating value measured with the calorimeter is better than 0.2%.Therefore, both methods correspond within the uncertainty of measurementof the calorimeter (0.3%). No significant drift of the measurementsignal was observed during the entire field test which was carried overa period of four months.

[0058]FIG. 6 shows a schematic of the inventive device. Part of a streamof natural gas is withdrawn from a main transmission line 1 and passedthrough a pressure-reducing device 2 into a branch line 3 at approx.20-100 mbar, i.e. substantially reduced to atmospheric pressure, andthen passed into an arrangement of sensors 4. The arrangement of sensors4 consists substantially of a source of infrared radiation, which is notshown, and two radiation detectors 5, 6 assigned to the source ofradiation. A third sensor 7 measures the thermal conductivity. Thesensors 5, 6, 7 continually record the measurement signals which aredirectly evaluated by an electronic evaluation unit 8 (conventionalprinted circuit).

[0059] The temperature is measured in a manner not shown so that it ispossible to convert the measured values to reference conditions. If thetemperature fluctuates sharply in the measurement environment, it isadvantageous to set or adjust the temperature to a figure of for example50° C.

1. A method of determining the properties of a combustible gas, inparticular of natural gas, said combustible gas consisting of aplurality of different components or different groups of components andhaving a thermal conductivity, said method comprising the followingsteps: a) at least a part of the combustible gas is exposed to infraredradiation and the amount of infrared radiation absorbed by thecombustible gas is recorded at least for a first wave length or spectralrange and a second wave length or spectral range, thus providing a firstand a second measurand, wherein the first and second wave lengths orspectral ranges are different, b) the thermal conductivity of thecombustible gas is recorded, thus providing a third measurand and c) thegas properties of the combustible gas are determined from the at leastthree measurands.
 2. The method according to claim 1, wherein thecombustible gas contains hydrocarbons and carbon dioxide and wherein thefirst and second wave lengths or spectral ranges are chosen so that theamount of hydrocarbons is detected from the first measurand and theamount of carbon dioxide is detected from the second measurand.
 3. Themethod according to claim 2, wherein the amount of infrared radiationabsorbed by the combustible gas is recorded for the wave lengths ofapprox. 3.5 μm and 4.3 μm.2.
 4. The method according to claim 2, whereinthe combustible gas contains methane and wherein the amount of infraredradiation absorbed by the combustible gas is also recorded for a thirdwave length or spectral range thus providing a fourth measurand fordetermining the properties of the combustible gas.
 5. The methodaccording to claim 3, wherein the third wave length is approximately 7,9μm.
 6. The method according to claim 1, wherein the amount of infraredradiation absorbed by the combustible gas as well as the thermalconductivity are recorded under reference conditions in a commonmeasuring environment.
 7. The method according to claim 1, wherein thetemperature and the pressure are recorded in step a) or b).
 8. Themethod according to claim 1, wherein the temperature and the pressureare kept constant in step a) or b).
 9. The method according to claim 1,wherein the gas properties are determined in step c) according to theformulae (6), (3), (1), (7) (8.1-8.9) and (9) in accordance with FIG. 3.10. The method according to claim 9, wherein the coefficients.a₀,a₁,a₂.and c₀,c₁,c₂. are determined for the equations (3) and (6)from the measurands on the basis of steps a) and b) with severalreference gases of known gas properties.
 11. A method of determining theamount of energy in a combustible gas having a gross heating value asone of its gas properties, in particular in natural gas, wherein thegross heating value is determined by the method of determining the gasproperties according to the invention, wherein a volume flow of thecombustible gas is passed through a volume flowmeter for measuring thevolume flow, and wherein the amount of energy is determined from thegross heating value and the volume flow.
 12. A device for determiningthe gas properties of a combustible gas, in particular of natural gas,wherein the combustible gas is fed into an arrangement of sensors (4)which comprises a source of infrared radiation and at least tworadiation detectors (5, 6) assigned to the source of radiation as wellas a sensor (7) to record the thermal conductivity and wherein thesignals of the arrangement of sensors are transmitted to an evaluationdevice in which the gas properties are determined by means of acorrelation.
 13. The device according to claim 12, wherein an opticalfilter is assigned to each of the radiation detectors (5,6), wherein theoptical filters are chosen such that the amount of hydrocarbons and theamount of carbon dioxide is detected.